Scoop diffuser for use on ram jets for securing isentropic compression of a supersonic stream



R. S. SCOOP DIFFUSER FOR USE ON RAM JETS FOR SECURING ISENTROPIC COMPRESSION OF A SUPERSONIC STREAM FIG! Filed Feb. '7, 1950 2 Sheets-Sheet 1 23 l 24 FUEL u I FIG. 2

FIG. 4

INVENTOR.

RANDOLPH S. RAE

ATTORNEY R. S. RAE

April 3, 1962 SCOOP DIFFUSER FOR USE ON RAM JETS FOR SECURING ISENTROPIC COMPRESSION OF A SUPERSONIC STREAM 2 Sheets-Sheet 2 Filed Feb. 7, 1950 FIG. 6

AIR FLOW FIG. 7

Prandll-Meyer, Expansion over a corner. Flow 01 sonjc velocity aqd parallel to the surface at m. 6 I Flow at supersamc velocity and parallel to the surface of INVENTOR.

RANDOLPH S. RAE BY ATTORNEY nite 3,027,711 Patented Apr. 3, 1962 3,027,711 SCOOP DIFFUSER FOR USE ON RAM JETS FOR SECURING ISENTROPIC COMPRESSION OF A SUPERSONKI STREAM Randolph S. Rae, Silver Spring, Md., assignor to the United States of America as represented by the Secretary of the Navy Filed Feb. 7, 1950, Ser. No. 142,727 4 Claims. (Cl. 60-356) The present invention relates in general to guided missiles, and more specifically to an improvement in air intakes for ramjet driven missiles, operating at supersonic speeds.

The diffuser intakes of ram jet missiles flying at velocities that are relatively high in comparison with the velocity of sound, have generally been of the Oswati-tsch convergent-divergent central duct design. A paramount objection to this type of diffuser intake is its inherent high drag. To approach isentropic compression a supersonic diffuser must provide a converging stream in the supersonic part, and a divergent stream in the subsonic part of the diffuser.

Other objects of the present invention are to provide a ram jet intake having the full isentropic compression of the supersonic stream, to provide a ram jet diffuser intake with suitable flow characteristics at nearly isentropic pressure conditions, and to achieve the above conditions without producing excessive drag.

A specific object of the present invention is to provide a ram jet diffuser intake positioned to occupy a minimum of space, thereby providing room in the major portion of the missile for greater fuel supply, pay load, and instrumentation.

Other objects and many of the attendant advantages of the invention will be readily understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is an elevational view of a ram jet missile embodying the invention;

FIG. 2 is a fragmentary axial section of the ram jet missile embodying the invention;

FIG. 3 is a transverse section of the ram jet missile according to the present invention, in plane 3-3 of FIG. 1;

FIG. 4 is a section through the scoop, in the plane 4-4 of FIG. 2;

FIG. 5 is 'a transverse section in the plane 5-5 of FIG. 2, showing diffuser details;

FIG. 6 is a diagrammatic section, illustrating the characteristics of a scoop diffuser embodying the present invention; and

FIG. 7 is a diagram illustrating the Prandtl-Meyer theory of expansion of supersonic flow around a corner.

Referring to the drawing and particularly to FIG. 1, there is shown a ram jet missile 10 embodying near its rear end the scoop diffuser or air chute 11 which constitutes the present invention. This scoop diffuser 11 is located on the under side of the missile forward of the combustion chamber, cooperates therewith, substantially as shown, and leaves unobstructed the entire forward portion 12 of the missile for space for fuel, pay load and instrumentation.

The scoop dilfuser 11 is positioned on the underside of the ram jet, where under ordinary flight conditions the missile will have a positive angle of attack, resulting in a thin boundary layer at the junction of the scoop and missile body, and thereby minimizing detrimental aerodynamic interference. Mach number limitations on the design efiiciency of the scoop diffuser are eliminated, since the angle at which the flow enters the body has no narrow limits.

Within the scoop diffuser 11 supersonic compression during flight is achieved by the reversal of the Prandtl- Meyer expansion of supersonic flow around a corner, illustrated in FIG. 7. This is made possible by a longitudinally curved base surface 13 of the scoop 11. Side plates 14 that cover only the portion of the stream within the Mach angle 15, namely, the angle whose sine is the reciprocal of the Mach number, are aflixed to the scoop as shown in FIG. 2. Under optimum operating conditions supersonic flow exists at 16 ahead of the point of intersection of the scoop and missile body, while behind this point, as at 17, subsonic flow supplies air to the combustor 19.

FIG. 7 shows the idealized isentropic flow conditions existing at a flow around a corner, Wherea supersonic flow 6 is converted into sonic flow a, under reversible conditions, without loss in either direction of conversion, according to the Prandtl-Meyer expansion. The scoop diffuser of the present invention is based on such conversion, which is made possible by the curved bottom surface 13 of the scoop. In the specific case where the supersonic velocity of the vehicle is twice that of sound, the Mach number is 2, and hence the sine of the Mach angle is 0.5, the reciprocal of 2, which corresponds to the Mach angle of 30. The scoop used as an illustration in the present case is designed arbitrarily for a vehicle operating at Mach 2, and hence the ,angle 15 shown in FIG. 6 is 30.

While specific details of the internal structure of the ram jets are relatively unimportant as far as the present invention is concerned, for completeness such structure is disclosed briefly as follows:

Referring to FIGS. 2 to 5, it will be seen that the air intake passage within the ram jet increases in cross-sectional area from inlet region 17, where it is a relatively narrow rectangle as shown in FIG. 4, to a substantially square shape at 20, shown best in FIG. 5, thus decreasing the air speed at 20 to approximately one-half the speed at 17.

From region 20 to region 21 at the outlet end of the air passage, the shape of the cross section changes from square to circular, so that a more nearly uniform distribution of the air results at the combustor 19. Fuel is supplied to said combustor from tank 22 through conduit 23, and burning takes place beyond the fuel discharge nozzles 24.

Pressure increases, due to instability within the combustor, in the specific example, will cause shock waves at a greater angle than 30 to be generated temporarily forward of the diffuser throat, as indicated in dash-dot lines at 18 in FIG. 6. However, as this large-angle shock wave moves forward, air spills or escapes laterally over the side plates 14, thereby re-establishing the supersonic flow regime in the throat area as soon as such spillage causes the pressure in the subsonic part of the difluser to fall below the maximum steady state value. This is an important feature of the scoop diffuser, in that provision for excess diffuser pressure leakage thus becomes a function of the pressure involved, and hence serves to stabilize the diffuser pressures. The scoop diffuser is thus an ideal diffuser in that no air flow is needlessly lost after the shock has passed to the diffuser throat.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In a ram jet device for use at supersonic speeds, a diffuser chute of rectangular, four-walled construction extending from the interior of the device to the exterior through a rectangular aperture in the wall of the device,

the chute having two opposite, normally vertical walls and two opposite, transverse walls situated generally one above the other, the upper transverse wall residing only within the device and extending to a forward transverse edge of said aperture, the lower transverse wall extending outwardly and forwardly of the aperture and ending in a transverse leading edge substantially forward of the aperture, the vertical walls having leading edges that extend from their respective ends of the leading edge of the lower wall to the respective ends of the forward transverse edge of the aperture.

2. In a device for use at supersonic speeds, an air-intake chute of four-walled construction extending from the interior of the device to the exterior through an aperture in the wall of the device, the chute having two opposite, normally vertical walls and two opposite, transverse walls situated generally one, above the other, the upper transverse wall residing only within the device and extending inwardly from a forward, transverse edge of said aperture, the lower transverse wall extending outwardly and forwardly of the device and terminating in a transverse leading edge situated forwardly of the aperture, the vertical walls having leading edges that extend from the leading edge of the lower wall to the respective ends of the forward transverse edge of the aperture, the leading edges of the vertical walls being inclined at an angle to the direction of the neighboring portion of the wall of the device, said angle being approximately complementary to the Mach angle at the air speed for which the device is intended.

' 3. In a device for use at supersonic speeds, an air-intake chute extending from the interior of the device to the exterior through an aperture in the wall of the device, the

chute having two opposite, transverse walls situated generally one above the other, the upper transverse wall residing only within the device and extending inwardly from a forward, transverse edge of said aperture at an obtuse angle to the adjacent portion of the wall of the device, the lower transverse wall extending outwardly and forwardly and terminating in a transverse leading edge forwardly of the aperture, the lower transverse Wall curving smoothly from said leading edge to a substantial distance within the device whereby Prandtl-Meyer flow takes place in the chute.

4. An air intake device for use with a supersonic ram jet vehicle having a combustor, said device comprising a scoop communicating with said combustor through an aperture in the wall of said vehicle and extending outwardly from said aperture to an opening for receiving air, said scoop having an end edge located forward of said aperture and straight side edges extending forwardly from the forwardmost end of said aperture at an acute angle to the wall of said vehicle and intersecting with said end edge.

References Cited in the file of this patent UNITED STATES P TENTS D. 143,822 Johnson Feb. 12, 1946 D. 155,404 Ebel et a1 Oct. 4, 1949 1,069,694 Hayot Aug. 12, 1913 2,352,790 Jordan July 4, 1944 2,439,273 Silvester Apr. 6, 1948 2,474,143 Forsyth June 21, 1949 2,503,973 Smith Apr. 11, 1950 2,632,295 Price Mar. 24, 1953 

